A magnetic field generator includes a refrigerating machine, a cold head, a superconductor which is formed in a cylindrical shape, a cold head extension portion which extends from the cold head and is brought into thermal contact with the superconductor at its extended end; and a vacuum heat insulating container having an internal space in which the cold head, the cold head extension portion, and the superconductor are received. The superconductor has a room temperature bore space, which is formed on its inner peripheral side along an axial direction of the superconductor, and is spatially isolated from the internal space of the vacuum heat insulating container. The room temperature bore space has both ends communicating to an outside of the magnetic field generator.